This invention relates to a magnetic recording medium, and more specifically to a magnetic recording medium using a hard ferromagnetic powder, especially chromium dioxide (CrO.sub.2), as a magnetic material.
Magnetic recording media for audio, video, or computer applications run, when recording or reproducing signals, in severe friction contact with magnetic heads. The magnetic recording medium whose magnetic layer uses a hard ferromagnetic powder, especially ferromagnetic CrO.sub.2 powder, causes wear of the head rather faster than the media that rely upon ferromagnetic .gamma.-Fe.sub.2 O.sub.3 powder, with or without cobalt doping or coating on the surface. In view of this, when a hard ferromagnetic powder such as of CrO.sub.2 is to be employed for the magnetic layer, means for reducing head wear and attaining good durability is required. To meet this end, it has been proposed to incorporate a nonmagnetic powder softer on Mohs scale than ferromagnetic CrO.sub.2 powder as an additive into the magnetic layer. Patent Application Public Disclosure No. 7620/1985, for example, teaches allowing the magnetic layer to contain, besides ferromagnetic CrO.sub.2 powder, additive powders of zinc oxide, .alpha.-iron oxide, nonmagnetic chromium oxide, silica and the like in a combined amount constituting from 20 to 50% by volume of the total powder amount. The additive powders are meant for reduction of head wear. However, while the addition improves head wear, the considerable amount required can sometimes reduce electromagnetic transfer characteristics or accelerate fouling of the head. Where the ferromagnetic powder is .gamma.-Fe.sub.2 O.sub.3 or the like, alumina or other powder harder on Mohs scale than CrO.sub.2 is often added. The use of such an additive powder in combination with CrO.sub.2 is, of course, not advisable because it promotes head wear. U.S. Pat. No. 4,015,042 discloses a technique of adding an inorganic powder high on Mohs scale, e.g., alumina, besides a ferromagnetic CrO.sub.2 powder or the like, to the magnetic layer, in an amount of about 2.5 to about 9% on the basis of the ferromagnetic powder amount. The technique is improper in that it increases head wear. As will be described in more detail later, the present inventors previously advocated in their copending Patent Application No. 73742/1989 the addition of 0.1 to 1.0% by weight of alumina, or far less than believed practical in the past. The concept helped alleviate the head wear and fouling (accumulation of wear products) problem to a considerable extent, but was still not a panacea for head wear.